Sound cards (and motherboards that can output in surround-sound audio) also feature a number of analogue Walkman-style (3.5mm) jacks at the rear of the card. These jacks generally comprise of headphone, line-out, line-in and microphone connectors. Many of these jacks can do double-duty by allowing the computer to adjust what they're used for (better known as jack sensing). Jack sensing, allows the sound card to automatically detect what's plugged into each port and assign the audio accordingly. For example, if you wanted to connect a set of headphones, a line-in and a microphone to these connectors, the computer would automatically detect what's there and assign each jack a function automatically. If they are to be used in a surround-sound configuration, the microphone port might be reassigned to handle rear-speaker output.
Just about all sound cards also include a MIDI (Musical Instrument Digital Interface) port on the rear of the card that also doubles as a joystick adaptor and some specialised sound cards also include a dedicated MIDI IN and OUT port - sometimes more than one of each. Many keyboards and digital music hardware devices use MIDI to communicate directly with a PC. It allows computer musicians to sequence music using the computer to store audio information.
Musicians looking at buying a sound card should check a sound card's drivers support ASIO (audio stream input output). If not, you'll get high latency - a delay between you pressing a MIDI keyboard key, software telling the sound card how to respond and the note being heard. This is a significant problem with multiple tracks, particularly when laying down a new tune while listening to recordings. ASIO speeds up communication so sounds occur exactly when they should; ASIO 2 reduces latency still further. Your PC's performance will also affect latency, but as a rule of thumb for a music-orientated sound card, a rating of 5ms or less is what you should aim for.
One point to watch out for if you're interested in home recording or realtime chatting is duplexing (the ability to record/and or input and play back sound simultaneously). These days, just about all mid to high-end sound card features full-duplex capabilities, meaning they can both play back data and record at the same time or play audio from multiple programs simultaneously. Watch out for any cards that offer half-duplex operation, as they are limited to performing one function at a time, making VoIP conversation or serious recording impossible.